I suspect it doesn't matter. A file that contains only englishEnglish text is often ASCII, and then there's just no difference between (unmarked) UTF-8 and ASCII/ANSI.
If you want to force the file to be UTF-8, you need to save it as UTF-8 with BOM. If there's no BOM ("Byte Order Mark", a special marker at the beginning of the file), the editor has to guess, and when there are no special characters in the file (e.g non-english-diacritics such as ä, ö or ê) it just doesn't matter, as the first 128 letters of all common character tables are equal.